A lithium sulfur battery is a lithium battery using sulfur element as a cathode electrode and using lithium metal as an anode electrode. According an amount of energy that a complete conversion from unit mass of elemental sulfur to S2− can provide, sulfur has a theoretical discharge capacity per mass of 1675 mAh/g, and a theoretical energy density of about 2600 Wh·kg−1, which is much higher than that of a commercialized conventional lithium ion battery (energy density is about 150 Wh·kg−1). In addition, sulfur is inexpensive and has no pollution to the environment. The lithium sulfur battery is expected to become a high energy density and long life secondary battery in the future.
However, in practical applications, lithium sulfur batteries still need to solve many problems, such as the electrical conductivity of the cathode electrode material, the volume change during charge and discharge, the lithium dendrite and the shuttle effect of intermediate products, which affect the cycle performance.
Like conventional lithium battery, the lithium sulfur battery has problems such as fire and explosion under a condition of battery abuse. As an important part of the lithium sulfur battery, electrolyte liquid plays an important role in the safety problems. Most of the electrolyte liquids in the lithium sulfur batteries contain ethers, such as 1,2-dimethoxyethane and 1,3-dioxolane, or carbonate esters, such as ethylene carbonate and propylene carbonate, as organic solvents. These solvents have low boiling points, being flammable and explosive. In recent years, developing flame-retardant electrolytes becomes a major measure to solve the safety problems of the lithium sulfur batteries, such as using phosphorus containing flame retardant additives in the electrolyte liquid, or fluorinating the solvents, to solve the flammable problem.